Avoid logging mulitple times for performance gains.

Before: 
![screenshot from 2014-07-07 09 29 06](https://cloud.githubusercontent.com/assets/4335742/3490656/21041f8c-0576-11e4-8c75-83929fb09767.png)

After: 
![screenshot from 2014-07-07 09 29 12](https://cloud.githubusercontent.com/assets/4335742/3490657/2148180e-0576-11e4-9847-19daaa1b5850.png)

@senny what do you mean by the log format? I verified it by looking at the output from the log file.

@matthewd I noticed some discrepancy with how we log messages in [ActionMailer::LogSubscriber](https://github.com/rails/rails/blob/21a49ffe9889a351882e8e97fc45790717b5a156/actionmailer/lib/action_mailer/log_subscriber.rb). xD yak shaving alittle here.